You could have looked at the NFRC sticker when they were put in. It usually lists what glass and gas are in the window. Also, the u-value listed could be a givaway all by itself. If you have a u-value of .30 - .35, you probably have low-argon.

You could also ask your dealer for a copy of the acknowledgement he got from the factory. Tell him you would like it for your records in case a warranty issue ever pops up and heaven forbid he is not around anymore. He will probably blank out the pricing he pays, but I'm sure he would be happy to make you a copy. I do it all the time for customers.

The last way to tell is to buy a $10,000.00 GasGlass tester. This will tell you conclusively one way or the other, but as you can see it's a little expensive.

Another way to try to discern if your glass has Low E and Argon gas in it is to buy a $49 infrared thermometer. I got one from RadioShack. It measures surface temperature of whatever its invisible beam strikes. At 0° outside, you should expect to get a reading of 57° center of glass if it has Low E and argon, instead of 44° without. That's assuming it gets that cold in your climate. Another method is to compare your glass temperature with a neighbor's when it is cold.